Multilayered polyamide film with excellent processability

ABSTRACT

The present invention provides a multilayered polyamide film having at least two polyamide resin layers, which develops less than 10 pinholes when evaluated for pinhole resistance by flexing it 1,000-times at 25° C. and which has an elongation of 6 mm or less at 120° C. in vertical pitch evaluation. This film has excellent resistance to pinholes and is well suited for lamination.

TECHNICAL FIELD

The present invention relates to a multilayered polyamide film withimproved resistance to pinholes, particularly to a multilayeredpolyamide film which maintains a stable pitch in the running directionduring printing or lamination and is well suited for lamination.

BACKGROUND ART

Conventionally, multilayer films containing a polyamide resin have beenwidely used in various fields because of their gas barrier properties,toughness, etc. The market requires further improvement of filmtoughness, especially the resistance to pinholes.

To realize further improvement in pinhole resistance, however, thepolyamide resin layer needs to be softened. Softening of the polyamideresin layer tends to cause the film to stretch in the running direction(the machine direction) during printing or lamination. Consequently, aproblem arises regarding pitch accuracy in the machine direction.

DISCLOSURE OF INVENTION

To solve the problem of the prior art, the inventors focused on therelationship between the softness of the film and the amount of stretchin the machine direction, and achieved the present invention. A primaryobject of the present invention is to improve pinhole resistance andpitch accuracy in the machine direction, thus achieving improved gasbarrier properties and more stable processability compared to the priorart.

The present invention is characterized by a multilayered polyamide filmcomprising at least two polyamide resin layers, the film havingexcellent processability such that the film develops less than 10pinholes when evaluated for pinhole resistance by flexing it 1000 timesat 25° C., and has an elongation of 6 mm or less at 120° C. in pitchaccuracy evaluation in the machine direction.

The multilayered polyamide film of the invention comprises at leastthree layers comprising in order of a polyamide resin layer, asaponified ethylene-vinyl acetate copolymer layer and a polyamide resinlayer, or at least three layers comprising in order of a polyamide resinlayer, a xylylene diamine polyamide resin layer and a polyamide resinlayer.

The multilayered polyamide film of the invention comprises at leastthree layers and may have, for example, a three-layer, five-layer orseven-layer structure. Examples of five-layer structure films include afilm consisting of a polyamide resin layer, a saponified ethylene-vinylacetate copolymer layer, a polyamide resin layer, a modified polyolefinadhesive resin layer and a linear low density polyethylene, and a filmconsisting of a polyamide resin layer, a xylylene diamine polyamideresin layer, a polyamide resin layer, a modified polyolefin adhesiveresin layer and a linear low density polyethylene. Examples ofseven-layer structure films include a film consisting of a linear lowdensity polyethylene, a modified polyolefin adhesive resin layer, apolyamide resin layer, a saponified ethylene-vinyl acetate copolymerlayer, a polyamide resin layer, a modified polyolefin adhesive resinlayer and a linear low density polyethylene, and a film consisting of alinear low density polyethylene, a modified polyolefin adhesive resinlayer, a polyamide resin layer, a xylylene diamine polyamide resinlayer, a polyamide resin layer, a modified polyolefin adhesive resinlayer and a linear low density polyethylene.

There is no specific limitation on the polyamide for forming thepolyamide resin layers of the invention. Examples of useful polyamidesinclude nylon 6, nylon 66, nylon 12 and copolymers thereof, nylon 6T/6I,nylon MXD-6 and the like. These polyamides may be used in combinationsof two or more. Further, in order to increase the pinhole resistance ofthe film, a modified ethylene-vinyl acetate copolymer may be added as acomponent for giving flexibility to the film.

The polyamide resin layer preferably comprises 99 to 85 wt. % of apolyamide and 1 to 15 wt. % of a modified ethylene-vinyl acetatecopolymer, more preferably 99 to 90 wt. % of the former and 1 to 10 wt.% of the latter, particularly 97 to 93 wt. % of the former and 3 to 7wt. % of the latter.

Examples of modified ethylene-vinyl acetate copolymers include (1)resins with partially saponified —OCOCH₃, (2) resins produced bypartially substituting —OCOCH₂CH₃ for —OCOCH₃ and (3) resins resultingfrom partial graft polymerization of an acid anhydride such as maleicanhydride.

There is no specific limitation on the saponified ethylene-vinyl acetatecopolymer. Examples of saponified ethylene-vinyl acetate copolymersinclude copolymers with an ethylene content of about 20 to 65 mole %,preferably about 29 to 44 mole %, and with a saponification degree ofabout 90% or higher, preferably about 95% or higher. There is nospecific limitation on the xylylene diamine polyamide resin. Examples ofuseful xylylene diamine polyamide resins include polymers synthesizedfrom m- and/or p-xylylene diamine and a dicarboxylic acid such as adipicacid.

The film of the invention may contain different kinds of polymers aslong as they do adversely affect the object of the invention and maycontain organic additives such as antioxidants, thermal stabilizers,lubricants, UV absorbents and the like in typical amounts.

The total thickness of the film of the invention is about 10-40 μm,preferably about 12-25 μm. The thickness of each of the polyamide resinlayers in the film, of which there are at least two, is about 3-15 μm,preferably about 5-10 μm. The thickness of the saponified ethylene-vinylacetate copolymer layer is about 2-10 μm, preferably about 3-10 μm. Thethickness of the xylylene diamine polyamide resin layer is about 2-10μm, preferably about 3-10 μm.

The flat polyamide multilayer film of the invention can be obtained, forexample, by co-extruding the resin layers from a T-die and superposingthe layers in an appropriate order onto a chilled roll where coolingwater is circulating. The film thus obtained is stretched to 2 to 4times its original size in the machine direction, for example, at 50 to100° C. using a roll stretching machine. Then the film is stretched to 2to 5 times its original size in the transverse direction at anatmospheric temperature of 90 to 150° C. using a tenter stretchingmachine. Subsequently, the film is thermally treated at an atmospherictemperature of 80 to 220° C. using the tenter stretching machine. Themultilayer film of the invention may be subjected to monoaxialstretching or biaxial stretching (simultaneous or sequential). Themultilayer film may be treated with corona discharge surface treatmenton one or both sides, if necessary.

With regard to pinhole resistance, the multilayer film of the inventiondevelops less than 10 pinholes when evaluated for pinhole resistance byflexing it 1,000 times at 25° C. Preferably, the number of pinholes is 6or less, more preferably 2 or less. Ten or more pinholes indicate nosubstantial improvement in pinhole resistance. If products are packagedwith such film, pinholes tend to occur during actual transportation.

The multilayer film of the invention has an elongation of 6 mm or lessat 120° C. in vertical pitch evaluation. Preferably, the amount ofelongation is 5 mm or less. When the amount of elongation is 6 mm orless in vertical pitch evaluation, the film maintains a stable pitch inthe machine direction during actual printing and lamination, so that noproblems will arise in the post-processing process of forming the filminto a bag. By contrast, when the amount of elongation is more than 6 mmin vertical pitch evaluation, the film is prone to stretch due to thetension applied during printing or lamination, resulting in a largedeviation and variation from the normal pitch. Such low pitch accuracycauses problems in the post-processing process of forming the film intoa bag.

The temperature of 120° C. was chosen for the vertical pitch evaluationbecause this is the maximum temperature to which films are usuallyexposed during actual printing and lamination.

The multilayer film of the invention has high toughness and excellentresistance to pinholes and is thus suitable for packaging heavy items,especially rice cakes, liquid items such as soups and the like.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described below in more detail withreference to Examples and Comparative Examples. The following methodswere used to measure the characteristic properties of the invention.

Pinhole Resistance Evaluation

Pinhole resistance was evaluated using a Gelbo flex tester manufacturedby Rikagaku Kogyo K.K., in the following manner. Each sample film wasformed into a cylindrical bag 150 mm in width when laid flat and 300 mmlong. The bag was attached to the Gelbo flex tester and flexed 1,000times at 25° C. at a torsional angle of 440° and with a twist and linearmotion of 15.0 cm. Then, using a penetrant, the number of pinholesformed on the central portion of the sample (measuring area: 300 cm²)was counted.

Vertical Pitch Evaluation

The vertical pitch was evaluated in the following manner. Each samplewas cut into long strips. Then 600 g of load was applied to the strip atan atmospheric temperature of 120° C. to measure the amount ofelongation of a 40 mm line drawn on the central portion of the sample.

EXAMPLE 1

A polyamide resin (a composition consisting of 96.0 wt. % of nylon 6 and4.0 wt. % of a modified ethylene-vinyl acetate copolymer), a saponifiedethylene-vinyl acetate copolymer (ethylene content: 32 mole %,saponification degree: 99%), and a polyamide resin (a compositionconsisting of 96.0 wt. % of 6-nylon and 4.0 wt. % of a modifiedethylene-vinyl acetate copolymer; trade name “NAV102X23”, manufacturedby Ube Industries, Ltd.) were coextruded from a T-die to form a flatfilm of three layers superposed in said order on a chilled roll whilecooling water was circulating. The film was then stretched to 3.0 timesits original size in the machine direction at 65° C. using a rollstretching machine and stretched to 4.0 times its original size in thetransverse direction at an atmospheric temperature of 110° C. using atenter stretching machine. Subsequently, the film was thermally treatedat an atmospheric temperature of 210° C. using the tenter stretchingmachine to form a 15 μm-thick film. The layers of the resulting film hadthicknesses of 6 μm, 3 μm and 6 μm in order from the surface.

EXAMPLE 2

A 15 μm-thick multilayer film was formed in the same manner as inExample 1 except that a composition consisting of 86.0 wt. % of nylon 6,10.0 wt. % of poly(m-xylylene adipamide) resin synthesized fromm-xylylene diamine and adipic acid, and 4.0 wt. % of a modifiedethylene-vinyl acetate copolymer was used in place of the polyamideresin used in Example 1. The layers of the resulting film hadthicknesses of 6 μm, 3 μm and 6 μm in order from the surface.

EXAMPLE 3

A 15 μm-thick multilayer film was formed in the same manner as inExample 1 except that poly(m-xylylene adipamide) resin synthesized fromm-xylylene diamine and adipic acid was used in place of the saponifiedethylene-vinyl acetate copolymer used in Example 1. The layers of theresulting film had thicknesses of 5 μm, 5 μm and 5 μm in order from thesurface.

Comparative Example 1

A polyamide resin (a composition consisting of 85.0 wt. % of nylon 6 and15.0 wt. % of an amorphous copolymer nylon comprisinghexamethylenediamine, terephthalic acid and isophthalic acid), asaponified ethylene-vinyl acetate copolymer (ethylene content: 32 mole%, saponification degree: 99%), and a polyamide resin (a compositionconsisting of 85.0 wt. % of nylon 6 and 15.0 wt. % of an amorphouscopolymer nylon comprising hexamethylenediamine, terephthalic acid andisophthalic acid) were coextruded from a T-die to form a flat film ofthree layers superposed in said order on a chilled roll while coolingwater was circulated. The film was then stretched to 3.0 times itsoriginal size in the machine direction at 65° C. using a roll stretchingmachine and stretched to 4.0 times its original size in the transversedirection at an atmospheric temperature of 110° C. using a tenterstretching machine. Subsequently, the film was thermally treated at anatmospheric temperature of 210° C. using the tenter stretching machineto form a 15 μm-thick film. The layers of the film had thicknesses of 6μm, 3 μm and 6 μm in order from the surface.

Comparative Example 2

A 15 μm-thick multilayer film was formed in the same manner as inComparative Example 1 except that a composition consisting of 75.0 wt. %of nylon 6, 20.0 wt. % of an amorphous copolymer nylon comprisinghexamethylenediamine, terephthalic acid and isophthalic acid, and 5.0wt. % of a modified ethylene-vinyl acetate copolymer was used in placeof the polyamide resin used in Comparative Example 1. The layers of thefilm had thicknesses of 6 μm, 3 μm and 6 μm in order from the surface.

Comparative Example 3

A 15 μm-thick multilayer film was formed in the same manner as inComparative Example 1 except that poly(m-xylylene adipamide) resinsynthesized from m-xylylene diamine and adipic acid was used in place ofthe saponified ethylene-vinyl acetate copolymer used in ComparativeExample 1. The layers of the film had thicknesses of 5 μm, 5 μm and 5 μmin order from the surface.

The multilayer films described in the Examples may optionally be treatedwith corona discharge surface treatment on one or both sides.

The physical properties of the multilayer films obtained in the Examplesand Comparative Examples are shown in Tables 1 and 2 below. The tablesshow data on the films obtained in the Examples and Comparative Examplesand treated on one side with corona discharge surface treatment.

TABLE 1 Example Exam- Example Test method Item 1 ple 2 3 and unitPinhole resistance 0 0 6 Number of evaluation pinholes Vertical pitch4.7 3.8 4.1 mm evaluation Processability A A A Test method 1 (Printing)Processability A A A (Dry lamination) Processability A A A (Extrusionlamination) Vibration test A A A Test method 2

TABLE 2 Comp. Comp. Comp. Test method Item Ex. 1 Ex. 2 Ex. 3 and unitPinhole resistance 20 0 25 Number of evaluation pinholes Vertical pitch6.3 7.1 3.8 mm evaluation Processability A B A Test method 1 (Printing)Processability A B A (Dry lamination) Processability C C A (Extrusionlamination) Vibration test B A B Test method 2

Test Method 1

Each sample film was processed using a printer and laminator. Theprinting pitch deviation was evaluated on an A to C scale.

A: No pitch deviation.

B: A pitch deviation occurs under certain processing conditions.

C: A pitch deviation occurs; the film is less suitable for processing.

More specifically, the test methods are as follows:

Processability (printing): an image was produced on the sample film at aprinting speed of 200 m/s using a gravure printer. The printing pitchdeviation was evaluated on an A to C scale.

Processability (dry lamination): a linear low density polyethylene filmhaving a thickness of 50 μm was dry laminated at a laminating speed of150 m/s using a dry laminator. The printing pitch deviation wasevaluated on an A to C scale.

Processability (extrusion lamination): A linear low density polyethylenewas extrusion laminated to a thickness of 50 μm at a laminating speed of120 m/s using an extrusion laminator. The printing pitch deviation wasevaluated on an A to C scale.

Test Method 2

Each laminated sample film was formed into bags (220 mm×320 mm) and 1 kgof rice cakes was packed into each bag. Ten bags containing rice cakeswere placed in one box (280 mm×370 mm×230 mm) and stacked in 5 rows of2. Then the box was placed on a vibration tester and vibrated for 30minutes. Thereafter, the sample was taken out and the occurrence ofpinholes was evaluated on an A to B scale.

A: No pinholes occur.

B: Pinholes occur; the film has poor pinhole resistance.

Table 1 shows that the films of the present invention have remarkablepinhole resistance and maintain stable vertical pitch accuracy in actualprinting and lamination. In contrast, as is evident from Table 2, thefilms obtained in the Comparative Examples have poor resistance topinholes and/or are not well suited for processing.

The present invention provides a multilayered polyamide film with thespecial property of excellent pinhole resistance, which has been greatlydesired in the market, and improved suitability for printing andlamination. This film can be widely used for a variety of purposes andis suited, for example, for food packaging, etc.

What is claimed is:
 1. A multilayered polyamide film comprising at leasttwo polyamide resin layers, the film having excellent processabilitysuch that the film develops less than 10 pinholes when evaluated forpinhole resistance by being flexed 1000 times at 25° C., and has anelongation of 6 mm or less at 120° C. in vertical pitch evaluation,wherein at least one of said polyamide resin layers comprises 97 to 93weight % of polyamide and 3 to 7 weight % of a modified ethylene-vinylacetate copolymer.
 2. The multilayered polyamide film according to claim1 which comprises at least three layers comprising in order a polyamideresin layer, a saponified ethylene-vinyl acetate copolymer layer and apolyamide resin layer.
 3. The multilayered polyamide film according toclaim 1 which comprises at least three layers comprising in order apolyamide resin layer, a xylylene diamine polyamide resin layer and apolyamide resin layer.
 4. The multilayered polyamide film according toclaim 1 wherein the polyamide is at least one member selected from thegroup consisting of nylon 6, nylon 66, nylon 12 and copolymers thereof,nylon 6T/6I and nylon MXD-6.
 5. The multilayered polyamide filmaccording to claim 1 which has a thickness of about 10-40 μm.
 6. Themultilayered polyamide film according to claim 2 wherein the saponifiedethylene-vinyl acetate copolymer contains about 20-65 mole % of ethyleneand has a saponification degree of about 90% or higher.
 7. Themultilayered polyamide film according to claim 3 wherein the xylylenediamine polyamide resin is poly(m-xylylene adipamide) and/orpoly(p-xylylene adipamide).
 8. The multilayered polyamide film accordingto claim 1 which is treated with corona discharge surface treatment onone or both sides.
 9. The multilayered polyamide film according claim 1,which has a thickness of about 12 to 15 μm.
 10. The multilayeredpolyamide film according claim 1, which comprises at least three layersof a polyamide resin layer, a saponified ethylene-vinyl acetatecopolymer layer and a polyamide resin layer in order of mention, thefilm having a thickness of about 12 to 15 μm, wherein polyamidecomprised in at least one of said polyamide resin layers is nylon 6 or amixture of nylon 6 and poly(m-xylylene adipamide).
 11. The multilayeredpolyamide film according claim 1, which comprises at least three layersof a polyamide resin layer, a poly(m-xylylene adipamide) resin layer anda polyamide resin layer in order of mention, the film having a thicknessof about 12 to 15 μm, wherein polyamide comprised in at least one ofsaid polyamide resin layers is nylon 6 or a mixture of nylon 6 andpoly(m-xylylene adipamide).